THis invention relates to turn-off circuits for a gate turn-off thyristor (GTO), and more particularly to circuits for providing a turn-off gate current pulse having a magnitude approximately proportional to the anode current.
The gate turn-off thyristor is a reverse blocking triode thyristor similar to the ordinary SCR in that the device is triggered into conduction by the application of a gating pulse which injects current into the gate lead, but dissimilar in that the anode current is turned off by a reverse gate pulse which withdraws current from the gate lead. DUe to the regenerative nature of the GTO device, which is also known as the gate turnoff switch or gate controlled switch, a relatively high reverse gate current is required to turn off the anode current. Typically a reverse gate current with a magnitude equal to at least 10 percent to 50 percent of the anode current is needed to obtain reliable turn-off. In practice, it is common to supply a preselected reverse gate pulse of sufficiently high magnitude to turn off the largest GTO anode current expected. In many cases the largest expected current is an overload current or even a fault current. Hence, in some circuits an unusually high turn-off gate current may be present even under full load conditions while at light loads the turn-off current may be several times larger than the anode current, thereby resulting in extremely inefficient operation.
An improved and more efficient circuit operation is achieved by a turn-off circuit which generates a variable magnitude reverse gate current more suitably matched to the magnitude of the anode current or load current to be turned off. While it is possible to sense the anode current directly, active approaches requiring knowledge of the anode current itself tend to be complicated and not suited for general use.